Malaria and HIV infection are two of the most important health challenges of our time. Children under 5 years of age and pregnant women are particularly vulnerable to the complicating effects of malaria and HIV co- infection. Treatment options within Africa have improved, with increasing availability of artemisinin-based combination therapies (ACTs) for malaria and expanded access to antiretroviral therapy (ART) for HIV. ACTs are critical for treatment of malaria due to widespread resistance to older drugs. ACTs exhibit complex pharmacology, undergo activation and/or metabolism via cytochrome p450 pathways and are susceptible to physiological differences and drug-drug interactions with ARTs. However, treatment guidelines have largely stemmed from adult studies, and have ignored the impact of age or pregnancy on drug disposition. Mounting evidence, including studies from our group, indicates that ACTs may be underdosed in children or pregnant women, and are associated with significant drug interactions and heightened toxicities, in particular neutropenia, in the setting of HIV infection and concomitant ART. Proper dosing is critical to improve treatment efficacy and minimize risk of drug resistance and toxicity. NIH-funded trials comparing ART-based treatment strategies for reducing malaria morbidity in HIV-infected children and pregnant women (PROMOTE) are currently underway in Tororo, Uganda, a region with high rates of malaria transmission and HIV infection. This proposal will complement PROMOTE and investigate the pharmacokinetics (PK) and pharmacodynamics (PD) of artemether-lumefantrine (AL), the most widely adopted ACT, in the context of ART, to optimize treatment for malaria and HIV. The specific aims are 1) To evaluate the impact of ART and age on the pharmacokinetics and pharmacodynamics of artemether-lumefantrine (AL) in HIV-infected children; 2) To evaluate the impact of ART and pregnancy on the PK and PD of artemether-lumefantrine in HIV-infected pregnant women; 3) To determine if ART and AL exposure following standard dosing is predictive of neutropenia in children and pregnant women with HIV and malaria co-infection. HIV-infected children and pregnant women, treated with either protease inhibitor (PI) or non-nucleoside reverse transcriptase inhibitor (NNRTI)-based ART will be co- enrolled from PROMOTE, and undergo intensive PK evaluations for AL during treatment for malaria. PI-based regimens include lopinavir/ritonavir, and NNRTI-based regimens include either nevirapine or efavirenz. Results will be compared to intensive PK evaluations in HIV-uninfected children also residing in Tororo. To assess associations between drug exposure and clinical outcomes, longitudinal population PK/PD studies will determine if changes in AL exposure impact malaria treatment efficacy, and if AL and ART exposure correlates with high rates of neutropenia observed in HIV and malaria co-infected individuals. The proposed pharmacology studies will inform future dosing guidelines for the most widely adopted ACT and ART regimens in Africa.